What is the most believable reason why someone might not accept a compliment?

not all compliments are .
the person giving the compliment said it with the wrong tone of voice.
compliments are sometimes mistaken as cynical remarks.
sometimes it's difficult to discern a person's sincerity.

the person giving the compliment used the wrong tone of voice

Explanation:

tone of voice is extremely important when talking, just take any phrase as an example (eg. I hate you) and think of how many different ways it could be interpreted, just simply by depending on the tone of voice used

The most believable reason why someone might not accept a compliment is the following one:

Compliments are sometimes mistaken as cynical remarks.

Explanation:

It happens occasionally due to the subjectivity of the person who is given the compliment, which might be viewed as a way to express sarcasm or irony. I remember an occasion at school when I told a girl she was beautiful, to which she replied angrily: "You're not that beautiful yourself!". I was astonished, since all I'd wanted to do was paying her a sincere compliment, but she thought I'd meant to say the opposite, that is, that she was ugly... Therefore, the most believable reason why someone might not accept a compliment is that compliments are sometimes mistaken as cynical remarks, that is, they are rejected due to misinterpretation.